Olefins (ethylene, propylene and butenes) production is a very energy-intensive process. Current steam cracking technology involves a process furnace to provide energy to crack feeds to olefin products, heat recovery from the products, a large compressor to pressurise the product stream to relatively high pressures (3-500 psig), and distillation to separate and purify the products. The process furnace is a relatively inefficient way to provide the heat of cracking: only about 40% to 50% of the heat released in the process furnace is used in the cracking reactions. The remainder of the furnace heat is recovered in the furnace convective section and integrated with the process gas heat recovery systems to provide high pressure steam to drive the reactor effluent and refrigeration compressors. Any additional energy (in the form of high pressure steam) is typically provided by auxiliary boilers. Steam cracking suffers from the disadvantage that providing compressor energy through such a steam cycle is thermodynamically inefficient, converting only about 25% of the thermal energy of the fuel into useful shaftwork. This, combined with the low efficiency of the process furnace, makes the production of olefins very fuel-intensive.
One way of improving the efficiency of such processes is to provide the heat for cracking by cogeneration (using both gas and steam turbines to provide energy), which is up to 55% efficient in converting fuel thermal energy into usable shaftwork. An example of this is disclosed in WO 01/04236, in which the steam cracking process is characterised in that the energy source to heat the hydrocarbon mixture is provided by a cogeneration unit. The cogeneration unit simultaneously produces thermal energy and mechanical energy by combustion of fuel provided from the cracked hydrocarbons; the mixture of hydrocarbons and steam are subjected to preheating by the thermal energy, whilst the mechanical energy is converted to electricity by an alternator or energy generator, which is then the used to heat the hydrocarbon mixture to the required cracking temperature.
U.S. Pat. No. 4,912,282 discloses a process for cracking hydrocarbons in which a mixture of fuel and an oxygen-containing gas is combusted in a cracking furnace to produce heat for cracking the hydrocarbons. The oxygen-containing gas comprises a mixture of preheated air and the expanded waste gas from a gas turbine, which has been fed with the product of combusting compressed air and fuel in a gas generator. The gas turbine drives electric generators, which power the compressors used in the process.
Whilst the use of gas turbines as described above improves the efficiency of such cracking processes, conventional steam-cracking processes are essentially self-sufficient in energy, with a significant proportion (for example 60% or more) of the total heat required for the whole process being available from the exhaust gases of the furnace and cooling of the cracked products. Accordingly the additional heat required, which is the part to which a cogeneration process can be applied, is relatively small (for example 40% or less), meaning that the overall energy efficiency gains available are also relatively limited, perhaps of the order of 15%.
Autothermal cracking is a new route to olefins in which the hydrocarbon feed is mixed with oxygen and passed over a catalyst. Combustion is initiated on the catalyst surface and the heat required to raise the reactants to process temperature and to carry out the endothermic cracking process is generated in situ. As a result, there is no need for a process furnace. Such a process is described for example in EP 332289B; EP-529793B; EP-A-0709446 and WO 00/14035.